In Perl, strings and integers are both represented by the same data type - scalars. (Internally Perl represents scalars as a C struct which has separate string and integer slots. But you don't normally need to worry about such concerns.) If you have a variable that contains an integer value, such as 23, and pass it to something that expects a string, like split, that integer will be silently converted to a string.

Here's exactly the same function I posted before, only now I'm using the integer 23 as the input...

use 5.010;
use strict;
use warnings;
# 23 is an integer.
#
say digit_sum(23);
# This expression is used to test that a string consists of a single
# digit. We use it a couple of different places, so we'll get define
# it once here.
#
use constant DIGIT => qr/^[0-9]$/;
# This is the function which adds digits
#
sub digit_sum {
my $string = shift;
# Otherwise, split into digits.
my @digits =
grep { $_ =~ DIGIT } # keep only the numeric characters
split '', $string; # split into characters
# Add the digits together.
my $sum = 0;
$sum += $_ for @digits;
# Handle the trivial case.
# If $sum is just a single digit, return it as-is.
return $sum if $sum =~ DIGIT;
# Otherwise, recurse.
return digit_sum($sum);
}